Electronic image sensors, such as charge-coupled device (CCD) or complementary metal oxide semiconductor (CMOS) pixel sensors, are replacing film as X-ray sensitive elements in dental and medical applications. Examples of use of CCD-type and other X-ray image sensors in dental and/or medical environments are described in U.S. Pat. Nos. 5,671,738 and 5,744,806, which are incorporated herein by reference.
In a typical configuration of an X-ray imaging system, the object to be imaged is placed between an X-ray generator and the image sensor. The image sensor registers the amount of radiation passing through the object. A concern in such an application of an X-ray sensor is to synchronize the image capture sequence of the image sensor with the output of the X-ray generator.
It is well known in the art that a dark signal accumulates in the sensor even when it is not exposed to X-rays. The dark current adds noise and reduces the dynamic range of the sensor. The dark signal is dependent on temperature and integration time. Since all electronic image sensors are subject to dark current, many techniques have been proposed to address the dark signal problem.
Methods of compensating for the dark signal in an X-ray image sensor are described in, for example, U.S. Pat. No. 5,519,437, which is incorporated herein by reference.
To minimize dark signal remnants, the sensor may be cleared of signal just prior to the onset of radiation and read out immediately after the end of the radiation pulse. Also, it is desirable to make the image acquisition period as short as possible.
Several approaches have been taken to address the synchronization concern. Methods known in the art for triggering an X-ray image sensor are described in, for example, U.S. Pat. No. 6,002,742, which is incorporated herein by reference.
In one known arrangement, the image sensor is electrically connected to the X-ray source and image acquisition is controlled by a timer of the X-ray source. The disadvantage of such an arrangement is that the image sensor can only be used together with a limited number of different X-ray sources since the connection between X-ray source and image sensor is not sufficiently standardized.
In a second arrangement, one or more supplementary X-ray sensing elements are located close to the imaging area to detect start and end of the X-ray pulse. A signal is sent from the supplementary sensing element to the control circuitry of the image sensor to control image acquisition. The use of supplementary sensing elements adds size and complexity to the image sensor arrangement. The supplementary sensing elements also have to cover a significant part of the image area to minimize the risk of being shaded by a dense part of the object to be imaged.
In a third arrangement, the image sensor itself is continuously read out while waiting for exposure. The signal, either from one pixel or summed from many pixels, is compared to a fixed or variable threshold to determine the onset of radiation. A disadvantage of this method, as used with, for example, CCD sensors, is that the readout is destructive and requires shifting of the image along the sensor. In arrangements with short exposure times the method causes loss of signal and image smearing. It is not possible to determine end of exposure using this arrangement in a system having an image sensor with destructive readout.